1. Field of the Invention
This invention relates to an improved method for measurement of serotonin uptake sites and a radioligand for use in such an analysis.
2. Description of the Background Art
Serotonin is known as a vasoconstrictor material, and is widely distributed in enterochromaffin cells, platelets, central nervous system and other tissues. In addition to the constrictory effects of smooth muscle of blood vessels, uterin, bronchium, intestine or the like, it has been known that serotonin plays an important role as a neurotransmitter in the central nervous system.
In cells that synthesize serotonin, tryptophan which is an amino acid is rapidly hydroxylated to 5-hydroxytryptophan by the enzyme tryptophan-5-hydroxylase, and is then decarboxylated to serotonin by the aromatic L-amino acid decarboxylase. It is known that serotonin is stored therein as a nondiffusable complex formed with adenosine triphosphate in platelets and enterochromaffin cells, and in synaptic vesicles of the central nervous system. Released serotonin undergoes oxidative deamination by monoamine oxidase to form 5-hydroxyindoleacetaldehyde. This is promptly degraded, mainly by further oxidation to 5-hydroxyindoleacetic acid by aldehyde dehydrogenase, and 5-hydroxyindoleacetic acid is excreted in the urine.
It is well known that termination of the effects of serotonin in synaptic function occurs in two ways by an uptake process and by metabolism of the transmitter. Although the details of serotonin metabolism have been clearly established, the process probably plays a minor role in terminating the action of serotonin at the synaptic cleft. The most likely terminating process is the reuptake of serotonin by the presynaptic terminal. Thus, serotonin uptake sites, which exist in the presynaptic nerve terminal, play an important role in regulating the serotonin content in the synaptic cleft. It is, therefore, of great interest to measure the serotonin uptake sites. Interestingly, it has been reported that there is a reduced density of serotonin uptake sites in the brain tissue of depressed patients and Alzheimer's disease. Study of serotonin uptake sites in the brains of people with these diseases is useful for the diagnosis and therapy of these diseases.
Radioreceptor assay has been used as measurement of the serotonin uptake sites. Radioreceptor assay is a method for measuring the radioactivity of a radioligand bound to sample after incubating the radioligand and sample. At present, radioligands such as tritium-labeled imipramine and tritium-labeled paroxetime have been used.
Radioligands used at present do not have high specificity and affinity for serotonin uptake sites in intact tissues, and tritium-labeled imipramine binds other binding sites as well as serotonin uptake sites. Therefore, it is necessary to develop an excellent radioligand having high specificity and high affinity for serotonin uptake sites.
Under the above-described circumstances, the present inventors have conducted extensive studies, and as a result, have found that tritium-labeled 6-nitroquipazine, or an acid thereof, has high specificity and affinity for serotonin uptake sites.